lavr: temporarily store custom matrix in AVAudioResampleContext

This allows AudioMix to be treated the same way as other conversion contexts
and removes the requirement to allocate it at the same time as the
AVAudioResampleContext.

The current matrix get/set functions are split between the public interface
and AudioMix private functions.

libavresample/options.c

@@ -84,13 +84,6 @@ AVAudioResampleContext *avresample_alloc_context(void)
     avr->av_class = &av_resample_context_class;
     av_opt_set_defaults(avr);
 
-    avr->am = av_mallocz(sizeof(*avr->am));
-    if (!avr->am) {
-        av_free(avr);
-        return NULL;
-    }
-    avr->am->avr = avr;
-
     return avr;
 }